This invention relates generally to lighting systems for hazardous atmospheres such as present in underground coal mining where explosive concentrations of methane and coal dust are possible hazards that must constantly be guarded against.
U.S. Federal Government regulations require that a coal face and mining machines working in the coal face area of underground coal mines be provided with lighting in order to reduce the risk of injury to miners working in these areas. There are specific minimum levels for both incident and reflected light which must be met in order to comply with the law.
The presence of methane gas and finely divided coal dust in coal mines create serious risks of explosions and fires where electrical equipment is used. All electrical equipment, including lighting systems, used in areas of coal mines which may contain explosive atmospheres must comply with U.S. Federal Government standards which are set out in applicable federal statutes found in 30 C.F.R. 18. These standards are concerned primarily with construction of "explosion proof enclosures", for electrical equipment, and "intrinsically safe circuitry".
Because of their greater efficiency, fluorescent lamps are employed almost exclusively in lighting systems for longwall mining and for mining machines. Many hundreds of fluorescent light tubes will be used at a typical longwall mining face. Each is protected in a heavy enclosure including rugged polycarbonate clear tubing which require special procedures and tools for removing and replacing individual components such as luminaires, bulbs, and control circuits.
Explosion proof enclosures for mine lighting power supplies have typically been "tailor made", with a variety of specially constructed, flame-proofed input and output access ways for high voltage AC conductors, low voltage DC intrinsically safe circuitry, switch actuators, etc. Thus, these previous explosion proof enclosures have often been manufactured a few at a time, on special order, without a standard design.